The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the UMTS (Universal Mobile Telecommunication Service) system, and LTE (Long term Evolution) is now under discussion as a next generation mobile communication system of the UMTS system. LTE is a technology for realizing high-speed packet-based communication that can reach a data rates of about 100 Mbps on the downlink and about 50 Mbps on the uplink. To this end, schemes and mechanisms are under discussions, for example, a scheme to reduce the number of network nodes in conventional UMTS networks. As an example, the base station in LTE, also known as eNB (enhanced Node B) or eNodeB, will perform the functions of a conventional radio access network (RNC) node and of a UMTS Node B. In addition, eNodeBs in LTE will interact directly with the core network and with other eNodeBs.
Irrespective of the wireless or mobile communication system used, radio link monitoring is vital to maintain radio connections. By regularly reporting the radio conditions to the system, different types of actions can be taken when radio link failure occur. In e.g. UTRA (UMTS Terrestrial Radio Access Network), the physical layers estimate the quality of the radio links and report, on radio frame basis, the synchronisation status to higher layers. The synchronization status are reported via so called synchronisation primitives which are described in greater details in the technical specification 3GPP TS 25.214 V7.4.0 entitled: “Physical Layer Procedures (FDD)”.
The mechanism of reporting radio link quality status is also specified in E-UTRA (Evolved UTRA), in which a fast and reliable detection of radio problems is considered essential in order to avoid unnecessary interference in uplink, waste of resources in downlink and unnecessarily long delays before e.g. reselection or handover of a UE can take place.
The radio link failure handling in E-UTRA (Enhanced-UTRA) is described in the technical specifications 3GPP TS 36.300 V8.1.0 entitled: “E-UTRA and Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Overall description; Stage 2”. The handling of radio link failure described in these specifications consists of two phases as illustrated in FIG. 1. As shown, the first phase is started upon radio problem detection which may lead to radio link failure detection after that e.g. a timer period has expired (the timer is denoted T1 in FIG. 1). In this first phase, user mobility is still controlled and managed by the network. This means the network can perform handover.
As shown, the second phase is started upon radio link failure detection which may lead to a so called radio resource control (RRC) state transition from a connected mode to an idle mode after the expiry of e.g. a timer, denoted T2 in FIG. 1. In this second phase, the network loses control over the UE. Therefore UE autonomously takes mobility related decision in accordance with the specified UE behaviour.
As mentioned earlier, the judgment (and reporting) of radio problem detection, as well as procedures for its reporting, is handled by the physical layer. The analogy with UTRA refers to the use of, the previously mentioned, synchronization primitives, e.g. the out of synchronization (out-of-sync) handling.
As an example, for E-UTRA downlink, the UE monitors radio link quality of the serving base station (or serving cell), in RRC connected (RRC_CONNECTED) mode, in order to indicate radio problems to higher layers. If the UE is not operating in a so called discontinuous reception mode (DRX mode), the physical layer in the UE checks, in every frame, the quality of the radio link measured over certain evaluation duration (e.g., 100 ms or 200 ms or any other suitable value) and compares against defined thresholds denoted Qin and Qout. When the radio quality determined by the UE is worse (or less) than the threshold Qout, the UE indicates radio problem or the so called out of synchronization to higher layers. The UE continues indicating radio problems until the quality becomes better than the threshold Qin. It is the higher layer(s) that triggers the start and stop of monitoring i.e. radio problem detection.
In addition to the above mentioned thresholds used to detect radio link problems, there are additional so called higher layer filtering parameters that can be used in order to further increase the reliability of radio link failure detection especially for the cases where the UE applies DRX and can avoid “ping-pong” between in-synchronization and out-of-sync. These parameters are known as hysteresis counters and timers. It should be noted that additional parameters and coefficients can also be used, but typically timers and counters are used. As an example, UTRA relies on timers and counters, denoted in the previously mentioned technical specification (3GPP TS 25.214), by N313 and N315. These are configured by higher layers i.e. via the network. They generally count the number of out-of-sync and in-sync indications. For E-UTRA, higher layer filtering parameters such as timers and counters are described in the technical specification 3GPP TS 36.331 V8.3.0 entitled: “E-UTRA Radio Resource Control (RRC); Protocol specification (Release 8)”. One of the timers mentioned in this technical specification and which relates to radio link failure detection and actions to be performed is denoted T310. It should be noted that additional timers are described in this technical specifications.
As mentioned before, the E-UTRA allows operation in DRX mode in RRC connected mode (also known as LTE_ACTIVE mode). DRX is an ongoing work on the LTE network (i.e. on UTRAN-LTE), and is a mechanism defined to save battery time and resources of a UE. With DRX a UE can turn on and off reception of layer 1/layer 2 (L1/L2) control in radio resource control connected state or connected mode, i.e. when the UE has established an RRC connection with the serving network.
In order to save battery time, the connected mode UE, while being in sleep mode during a predetermined DRX cycle period, wakes up at specific timings in order to check/monitor for possible control channels allocated by the LTE base station (i.e. eNB) to determine if there is data to receive. When there is no data to receive, the UE switches to the sleep mode and stays in the sleep mode until the next wake-up time. The control channel checked/monitored by the UE during the wake-up time is known as PDCCH (Physical Downlink Control Channel). When there is data to receive, the UE receives the data from the eNB and sends a response signal (ACK/NACK) indicating a successful or a failure in the reception of the data transmitted. As an example, DRX uses one or two predefined cycles (long and/or short cycles) at the beginning of which the UE should monitor the PDCCH over a certain amount of TTIs (Transmission Time Interval) under a so called Active Time. During the Active Time, the UE monitors the PDCCH for PDCCH-subframe(s). The number of consecutive PDCCH-subframe(s) at the beginning of the DRX cycle (i.e. during the Active Time) is known as the “On-duration Timer”. The On-duration timer in the beginning of each cycle also defines how long a UE should monitor the PDCCH and is also based on the system frame number (SFN), specified as an integer offset. The PDCCH can carry both downlink assignments as well as uplink grants scheduled by eNB. It should be noted that the same DRX mechanism is used both for the downlink (DL) and the uplink (UL).
Whether the UE is awake (i.e. monitors the PDCCH) or is asleep after the On-duration period depends on activity, i.e. possible receptions of PDCCH control data during that period. When the UE successfully decodes a PDCCH assignment or grant, it re(starts) the so called inactivity timer. The inactivity time extends the time during which the UE further monitors the PDCCH.
It should be mentioned that the network configures the DRX via the higher layers (i.e. by RRC). The UE can be configured to use long DRX cycle or the short DRX cycle or both. The UE always follow one DRX cycle at any moment even if two DRX cycles (i.e. short and long DRX) are configured. The network (i.e. higher/upper layers) can configure a DRX cycle between e.g. 2 ms and up to e.g. 2.56 seconds depending upon the type of service e.g. 2-20 ms for voice over internet protocol (VoIP) and e.g. 1-2 seconds for browsing on the Internet.
As mentioned before, when DRX is used, the UE tries to stay inactive as much as possible during the silent periods of the DRX cycle to save its battery. However, this also implies that the UE will mainly perform measurements at the wake up instances for e.g. mobility reasons; radio link problem detection (e.g. out-of-sync detection and in-sync detection) etc.
A drawback with this is that due to the insufficient measurement opportunities in DRX mode (depending upon DRX cycle) it is possible that the UE would be unable to promptly detect the radio link problem.
Furthermore, it is likely that a very large number of UEs are kept in DRX mode and the network can abruptly switch one or several UEs to operate in continuous reception mode in order to transmit data. Thus the UE(s) should stay well connected in terms of radio link quality and so any radio link problem(s) should be reported to the network promptly. In other words, it is important that the radio link problem detection is designed to work effectively in both DRX and non-DRX (i.e. continuous) modes of operation. But since the number of measurements samples, that are required to achieve estimation accuracy equivalent to non-DRX mode, can be relatively large, a UE operating in DRX mode may fail to promptly detect radio link problems due to the insufficient measurement opportunities (i.e. evaluation periods) by the UE in DRX mode.